The present invention relates to a method for improving the optical properties of filled paper. More particularly, the present invention relates to a method for improving the brightness of paper containing calcium carbonate by treating the filled paper with bleaching agents.
Papers produced according the process of the above invention allow papermakers to charge a premium price for the product, while obtaining improved optical properties without significantly increasing the cost of production.
The invention may be used on all grades of paper that contain filler. However, the invention is particularly useful on paper that is produced from mechanical pulps.
One of the most important characteristics of paper is the degree of whiteness of the paper. Generally, the whiter the paper, the higher the selling price. Efforts on the part of papermakers to obtain whiter papers have been directed to treatment of the pulp by subjecting it to bleaching agents such as alkaline hypochlorite. However, in bleaching the pulp, large quantities of bleaching agents are required. This contributes to increased production cost, as well as inefficient use and consumption of bleaching agents.
Papermakers who produce paper from mechanical pulps lose brightness due to alkaline darkening of the pulp when fillers such as calcium carbonate are used in the papermaking process. This darkening effect restricts the use of certain fillers, such as calcium carbonate, as well as reduces the price that can be charged for papers whose optical properties are less than desired. Because of the increased cost of using current bleaching techniques, a more economical bleaching technique is required. Additionally, because of the darkening that occurs in paper that contain mechanical pulps and fillers such as calcium carbonate, a technique to regain the brightness losses in this type of paper is required.
What has been found to be novel and unanticipated by the prior art is a process for bleaching papers containing mechanical pulps and fillers such as calcium carbonate, clay, and talc.
It is therefore an object of the present invention to provide a process for brightening filled paper. Another object of the present invention is to provide an economical process for the production of quality paper produced from mechanical pulps and calcium carbonate. Yet another object of the present invention is to provide a finished paper product that is particularly useful in applications where a high degree of whiteness is required. A further object of the invention is to allow the papermaker to increase the brightness of paper without the use of an expensive bleaching plant. A final object of the invention is to allow the papermaker to use calcium carbonate fillers and mechanical pulps for producing paper without suffering the effects of alkaline darkening.
These and other objects of the present invention will become apparent as further provided in the detailed specification which follows.
U.S. Pat. No. 2,150,926 discloses a process for bleaching sheet materials formed of fibrous pulp of vegetable origin by using hydrogen peroxide or substances generating hydrogen peroxide in solution.
U.S. Pat. No. 2,510,595 discloses bleaching of groundwood paper with an alkaline solution containing a per-compound, such as hydrogen peroxide, and a phosphate.
U.S. Pat. No. 2,613,579 discloses simultaneous bleaching and sizing of paper sheets with an alkaline aqueous solution of hydrogen peroxide and a wax dispersion.
According to Tang, xe2x80x9cStabilization of Paper Through Sodium Borohydride Treatmentxe2x80x9d, American Chemical Society, Pages 212, 427-441 (1986), reducing agents, such as sodium borohydride have been used to brighten paper.
None of the above referenced subject matter in whole or in part suggests that filled paper produced with mechanical pulps can be brightened through the use of various bleaching agents.
What has been found to be novel and unanticipated by prior art is a method for improving the optical properties of filled paper by bleaching the paper.
The process of the present invention is useful on grades of paper that contain filler. The invention is especially useful on grades of paper that are produced from mechanical pulps and calcium carbonate filler.
The process of the present invention is useful on grades of paper that contain filler. The present invention is especially useful on grades of paper that are produced from mechanical pulps and contain calcium carbonate.
Calcium carbonate fillers that are useful in the present invention include, but are not limited to, any calcium carbonate containing mineral, for example, limestone, chalk, dolomite, and synthetically produced precipitated calcium carbonate. Other mineral fillers useful in the present invention include talcs, clays, and synthetic silicon based fillers. The filled paper is produced using standard papermaking techniques well known in the art. The filled paper may contain from about 1 percent to about 40 percent by weight filler. The preferred weight of the filler in the paper is dependent upon the particular grade of paper being produced.
Bleaching agents useful in the method of the present invention may be any type of bleaching agent. Bleaching agents especially useful in the method of the present invention are selected from the group consisting of hydrogen peroxide, sodium borohydride, and sodium hydrosulfite. The bleaching agents may be applied as an aqueous solution containing from about 0.1 percent by weight to about 50 percent by weight bleaching agent. Preferably, from about 0.1 percent by weight to about 5 percent by weight based on the weight of the paper is used. More preferably, the level of treating agent is from about 0.1 percent by weight to about 2 percent by weight. The bleaching agent may be applied to the paper after it is produced, such as for example, at the size press or rewinders.
It is believed that when paper containing calcium carbonate is bleached with hydrogen peroxide, the alkalinity of the calcium carbonate activates the hydrogen peroxide, resulting in a greater improvement in brightness. What is being suggested here is a theory of why the method of the present invention allows for the bleaching of calcium carbonate containing paper. It should be accepted only as a theory and should not under any conditions whatsoever by employed to limit the scope of the present invention, which is further illustrated by the Examples which follow, and more specifically defined by the Claims.